Gas chromatography is essentially a physical method of separation in which constituents of a test sample in a carrier gas are adsorbed and desorbed by a stationary phase material in a column. A pulse of the sample is injected into a steady flow of carrier gas. At the end of the column the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern which, by calibration or comparison with known samples, indicates the constituents of the test sample. The main components of such a system are the column, an injector with a mixing chamber for introducing the sample into the carrier gas, a detector at the outer end of the column, gas controls and a device such as a computer for treating and displaying the output of the detector. An oven may be used to elevate temperature to maintain the sample in a volatile state, and to improve the discrimination of constituents.
In the use of open tube or packed capillary types of columns, only a small flow of carrier gas with the sample is desired, whereas it is more accurate and convenient to inject larger quantities of the sample. Therefore, a small portion of the gas mixture is bled into the column and the major portion is split off and vented. Such a system is known as a "split injection" system. The injector generally contains a septum through which sample is injected. The mixing chamber usually has an outlet for a purge gas that is a portion of the carrier gas passed along the septum. The purge gas removes vapors emitted from the septum during operation at elevated temperature, as the vapors otherwise could contaminate the carrier and its test sample flowing to the column.
An article "The Effects of Inlet Liner Configuration and Septum Purge Flow Rate on Discrimination in Splitless Injection" by J. V. Hinshaw, J. High Resolution Chromatography 16, 247-253 (Apr. 1993) illustrates several techniques for gas regulation. One is a forward-pressure design in which the carrier gas inlet to the injector is regulated at constant pressure, with mass flow being controlled in the outlet line of the split flow. Another is back-pressure regulated from an outlet line, with mass flow being controlled in the inlet line to the injector. The septum purge is effected through a restriction in the outlet line to maintain small purge flow and a selected pressure in the injector. The restriction may be fixed, or may be a needle valve for adjusting flows in other branches.
Pressure regulators used in gas chromatography are generally known, including older style mechanical devices that utilize spring loaded diaphragms. In newer systems electronic pressure sensors control variable restrictors for flow control to regulate pressure. In gas chromatographs, the pressure typically is generally detected in or proximate the injector. The restrictor of the regulator may be downstream in the same line, or in either of the inlet or split vent lines.
For flow rate controllers, U.S. Pat. No. 4,096,746 (Wilson et al), for example, discloses a mechanical flow controller that contains a diaphragm and a restrictor element in which pressure differential across the restrictor regulates the diaphragm for gas flow. In an electronic system, flow rate is detected by sensing pressure differential across a restrictor element, and the sensor controls an electrically variable restrictor. Heretofore, in current systems, the sensor and restrictor have been disposed in the same line.
A particularly desirable configuration for gas chromatography is the forward pressure design in which the carrier gas inlet to the injector is regulated at constant pressure, with the mass flow being controlled in the outlet line of the split flow. Benefits are improved performance and mass flow discrimination as indicated in the aforementioned article by Hinshaw. However, in this type of system, a mass flow controller including its sensor placed in the vent line has not been practical as it does not function properly in this location. One reason is that the mass flow sensor has a restrictor that creates a pressure drop substantially greater than the desired pressure at the outlet location, so that the back pressure at the injector would be too high. Another is that pressure drop across the restrictor (representing flow detection) is nonlinear in the desired low pressure range of the outlet location, whereas it is essentially linear at higher pressures. Therefore, flow rate in a forward pressure regulated system has generally been set manually by use of a needle valve in the split vent line.